A conventional utility knife comprises an elongated handle shaped to fit comfortably in an individual's hand, and a blade fixed to, and extending from one end of, the handle. A typical utility knife blade has a trapezoidal shape, with its cutting edge extending along the longer base of the trapezoid. The blade is usually replaceable, and when installed in the handle, extends therefrom in such a way that the shorter base of the trapezoid is fully or almost fully enclosed within the handle. Consequently, the exposed part of the blade is generally in the form of a trapezoid or a triangle.
Ordinarily, the handle consists of two similar parts that meet each other in a plane along which the blade extends. The parts are held together by one or more machine screws. One or more protrusions within one of the handle parts cooperate with notches in the shorter base of the trapezoidal blade to secure the blade against longitudinal movement relative to the handle when the handle parts are fitted together. The handle is also usually hollow, and the space inside the handle can be used to store spare blades.
The conventional utility knife handle has a tapered transition from the part that is grasped to the end from which the blade extends. A roofer who works with asphalt shingles typically makes extensive use of a utility knife to cut shingles, roofing paper, rubber waterproofing sheets, and other sheet roofing material. Slipping of the hand along the knife handle in the direction of the blade due to fatigue, moisture, inattention and other factors can result in laceration by the cutting edge of the blade or abrasion by contact with the hard granules embedded in an asphalt shingle. Other workers who use utility knives are subject to similar injuries.
Roofers, and other workers who use utility knives, frequently have occasion to cut a surface only to a limited depth, shorter than the distance by which the cutting edge of a typical utility knife blade protrudes from the knife handle. For example, gypsum roofing board used for fire protection, such as “DENSDECK DURAGUARD” roofing board, is cut to size by scoring along a straight line and then snapping the parts on both sides of the score line apart. To score such materials properly with a utility knife, the cutting depth of the knife blade should be relatively small, and in any event less than the thickness of the material.
Although some utility knives having retractable blades can be adjusted to set the cutting depth, a conventional knife in which the trapezoidal blade is secured against longitudinal movement by the engagement of protrusions within the handle with notches along its short base has no capability of rapid cutting depth adjustment. The handle parts must be separated. Then, after the blade-holding protrusion or protrusions are engaged with other notches in the blade, the handle parts must be reassembled. The knife is limited to a relatively small number of discrete cutting depths, corresponding to a whole number equal to or less than the number of notches in the short base of the trapezoidal knife blade.
It is also important, especially in roofing applications, to be able to cut shingles from the top by gripping a utility knife in such a way that the cutting edge faces in a direction away from the wrist, and also to cut other roofing materials such as rubber sheets from the underside by a pulling action in which the knife is gripped in such a way that the cutting edge faces in a direction toward the wrist. In the first of these instances, the knife handle is held in an oblique relationship to the work. In the second, the knife handle is generally perpendicular to the work. There is a risk of injury when operating a conventional utility knife in each of these two modes.